docs: require notify_closing before closing descriptors

[sricharanreddycheruku]

Problem

The notify_closing documentation says the close steps can be reordered whenever there are no checkpoints. Closing the underlying descriptor before notification is unsafe: on Linux, if the descriptor was duplicated, its epoll registration can survive the close and cannot be removed using the closed descriptor.

Change

Require notification before the actual close, retain the no-checkpoint requirement, and clarify that marking the object closed and notifying waiters may be reordered. Update the shared low-level reference, all three backend docstrings and their generated exports. Add a documentation newsfragment.

Addresses the ordering documentation described in #3520. Its remaining platform implementation issues are outside this documentation change.

Verification

• Linux epoll reproduction: close-before-unregister raises EBADF and leaves an event registered while a duplicated socket remains open; unregister-before-close leaves no stale event.
• python src/trio/_tools/gen_exports.py succeeds.
• sphinx-build -W --keep-going -b html docs/source ... succeeds without warnings.
• Sphinx lint, Black, Ruff and git diff --check pass for the changed files.
• AST comparison verifies all six changed Python files differ only in docstrings. Runtime tests were not rerun for this documentation change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (bcc9cee) to head (311d6ff).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3536   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19474        19474           
  Branches         1323         1323           
===============================================
  Hits            19474        19474           

Files with missing lines	Coverage Δ
src/trio/_core/_io_epoll.py	100.00000% <ø> (ø)
src/trio/_core/_io_kqueue.py	100.00000% <ø> (ø)
src/trio/_core/_io_windows.py	100.00000% <ø> (ø)

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